The invention relates generally to a power generator, and in particular to reduction of heat dissipation and undesirable voltage differentials in a power generator.
In order to improve generator efficiency and reduce generator size, high power electrical generator manufacturers are constantly endeavoring to improve generator thermal performance and efficiency. For example, a prior art design of a high power electrical generator 100 is illustrated in FIGS. 1 and 2. FIG. 1 is an end view of a cross-section of power generator 100 from an isometric perspective. FIG. 2 is a cut-away view of power generator 100 along axis 2-2. As shown in FIGS. 1 and 2, power generator 100 includes a substantially cylindrical stator 102 housing a substantially cylindrical rotor 110. Power generator 100 further includes multiple axially oriented keybars 118 that are circumferentially distributed around an outer surface of the stator 102. Each keybar 118 is mechanically coupled to the outer surface of stator 102. Each keybar 118 is further mechanically coupled at each of a proximal end and a distal end to one of multiple flanges 204 (not shown in FIG. 1). The multiple keybars 118, together with the multiple flanges 204, form a keybar cage around the stator 102.
An inner surface of stator 102 includes multiple stator slots 106 that are circumferentially distributed around an inner surface of stator 102. Each stator slot 106 is radially oriented and longitudinally extends approximately a full length of stator 102. Each stator slot 106 receives an electrically conductive stator winding (not shown).
Rotor 110 is rotatably disposed inside of stator 102. An outer surface of rotor 110 includes multiple rotor slots 114 that are circumferentially distributed around the outer surface of rotor 110. Each rotor slot 114 is radially oriented and longitudinally extends approximately a full length of rotor 110. An air gap exists between stator 102 and rotor 110 and allows for a peripheral rotation of rotor 110 about axis 130.
Each rotor slot 114 receives an electrically conductive rotor winding (not shown). Each rotor winding typically extends from a proximal end of rotor 110 to a distal end of the rotor in a first rotor slot 114, and then returns from the distal end to the proximal end in a second rotor slot 114, thereby forming a loop around a portion of the rotor. When a direct current (DC) voltage differential is applied across a rotor winding at the proximal end of rotor 110, an electrical DC current is induced in the winding.
Similar to the rotor windings, each stator winding typically extends from a proximal end of stator 102 to a distal end of the stator in a first stator slot 106, and then returns from the distal end of the stator to the proximal of the stator in a second stator slot 106, thereby forming a stator winding loop. A rotation of rotor 110 inside of stator 102 when a DC current is flowing in the multiple windings of rotor 110 induces electromagnetic fields in, and a passage of magnetic flux through, stator 102 and the loops of stator windings. The passage of magnetic flux through the stator windings induces a current in the stator windings and a power generator output voltage. The passage of magnetic flux through stator 102 induces eddy currents in the magnetically and electrically resistive stator. The eddy currents cause the dissipation of energy in stator 102 in the form of heat and impose a thermal constraint on the operation of generator 100.
FIG. 3 is a partial perspective of generator of 100 and illustrates a typical technique of constructing stator core 104. One known thermal management technique is the construction of stator core 104 from multiple ring-shaped laminations 302. As shown in FIG. 3, the multiple ring-shaped laminations 302 are stacked one on top of another in order to build up stator core 104. Each lamination 302 is divided into multiple lamination segments 304. Each lamination segment 304 includes multiple slots 120 (not shown in FIG. 3), wherein at least one slot 120 of each segment 304 aligns with one of the multiple keybars 118. Each keybar in turn includes an outer side 124 and an inner, or locking, side 122 that mechanically mates with one of the multiple slots 120. Stator core 104 is then constructed by sliding each lamination segment 304, via one of the multiple slots 120, into the keybar cage formed by the multiple keybars 118. The coupling of one of the multiple slots 120 of a lamination segment 304 with a locking side 122 of a keybar 118 affixes each lamination segment 304, and thereby each lamination 302, in position in stator 102. By building stator core 104 from stacked laminations 302, as opposed to constructing a solid core, circulation of a current induced in stator 102 is limited to a lamination, thereby restricting current circulation and size and concomitantly reducing stator heating. However, the above thermal management technique does not fully address the thermal problems caused by the coupling of magnetic fields into stator 102.
Furthermore, induced magnetic flux also passes through, and spills outside of, stator 102, coupling into each of the multiple keybars 118. The coupling of magnetic flux into a keybar 118 induces keybar voltages and keybar currents in the keybar, which current flows from the keybar to a flange 204 coupled to the keybar. A mechanical joint by which a keybar 118 is coupled to a flange 204 can be a poor electrical conductor that provides a high resistance path for the current. As a result, the joint can be a source of undesirable energy dissipation and heat generation in power generator 100, and is also a potential source of arcing and pitting in the power generator. Furthermore, the flow of keybar current in a magnetically and electrically resistive flange 204 results in undesirable energy and heat dissipation in the flange. To avoid overheating the joint and the flange 204 and potential arcing and pitting, a power generator such as power generator 100 sometimes must be operated at backed off levels of magnetic flux and output voltage, reducing the efficiency and rated power level of the power generator 100.
In addition, the induction of keybar voltage in each of the multiple keybars 118 can result in a voltage differential between keybar voltages induced in two of the multiple keybars 118. When adjacent keybars 118 are coupled to adjacent lamination segments 304, a keybar voltage differential appearing between the adjacent keybars 118 may also appear across the adjacent lamination segments 304. The voltage differential between adjacent lamination segments 304 can cause arcing between the two segments, overheating in the stator core 104, and reduced generator performance. The arcing can also create localized heating in the core, causing lamination segments 304, and lamination rings 302, to fuse together. Such fusing can spread quickly in generator 100 as the lamination segments 304, and lamination rings 302, short circuit to each other, resulting in damage to the generator.
Therefore, a need exists for a method and apparatus for further reducing the heat dissipated in the stator and for reducing keybar voltage differentials that may appear between keybars.
Thus there is a particular need for a method and apparatus that reduces the heat dissipated in a generator stator and that reduces keybar voltage differentials that may appear between keybars. Briefly, in accordance with an embodiment of the present invention, a thermal control and keybar voltage reduction mechanism is provided for use in a power generator having multiple keybars. The thermal control and keybar voltage reduction mechanism includes a keybar coupler capable of being electrically coupled to each of a first keybar of the multiple keybars and a second keybar of the multiple keybars. When the rotor rotates in the stator, the keybar coupler provides a low resistance electrical path from the first keybar to the second keybar for a current induced in the first keybar the rotation of the rotor. By providing a low resistance path, the thermal control and keybar voltage reduction mechanism shunts the current away from a high resistance path and reduces the heat dissipated by the power generator. In addition, by shunting the current away from a high resistance path, a voltage differential that can appear in the high resistance path is reduced, which reduces the likelihood of arcing and pitting in a power generator. Furthermore, by providing a low resistance path between two coupled keybars, the voltage differential reduction mechanism produces a larger current than would be produced in a single uncoupled keybar. The current in turn produces a first magnetic field that opposes a second magnetic field induced in the stator by the rotation of the rotor. By opposing the second magnetic field, the first magnetic field reduces the effective magnetic field induced by the rotation of the rotor, thereby reducing voltage differentials that can be induced by the effective magnetic field.